It is known to place the positive feeding devices between the yarn supply packages and the yarn selection unit. The latter unit is directly associated with the knitting machine, which is thus equipped with a number of yarn selection units corresponding to the number of knitting systems. A separate positive yarn feeding element is required for each yarn. This makes the knitting machine and its feeding system complicated. A major problem of these knitting machines specifically designed for the production of striped fabrics is that bulky equipment must be installed immediately adjacent the knitting system, thus making the machine difficult to assemble, to repair and to adjust.
The technical problem underlying this invention is to devise a method of the above-outlined type which permits of feeding of yarns of different colours to a conventional knitting machine, thus enabling a simple knitting machine to produce striped fabrics.
This problem is solved by the invention in that for feeding the yarns to a conventional knitting machine having no selection units immediately associated with its knitting systems, an unbroken yarn consisting of portions of different colours is formed for and fed into each knitting system of the machine by the following steps:
(a) in a separate selection unit controlled in synchronism with the working cycle of the machine and in dependence on the desired striping pattern a yarn of the desired colour is selected, PA1 (b) the selected yarn is connected at a joining station to the unbroken yarn travelling into the knitting machine, PA1 (c) an intermittent buffer store is formed of the unbroken yarn downstream of the selection unit, and PA1 (d) the unbroken yarn is positively fed from the buffer store to the knitting system, the lengths of unbroken yarn being controlled in accordance with the desired striping pattern.
By use of the method according to the invention it is possible to produce striped fabrics on a conventional knitting machine with relatively simple additional equipment, which can be located at a certain distance above or outside of the knitting machine itself. The feeding system is also simpler than in known cases, because only one unbroken yarn has to be fed to each knitting system.
The control of the length of unbroken yarns running into each knitting system preferably starts at the joining station. This enables the control to place the joints of the yarn ends, which are normally knots, at a specific location in the fabric where they do not disturb the overall appearance, e.g. at the location where the fabric is later severed.
The invention also relates to a knitting machine for producing striped fabrics from yarns of different colours stored in yarn supplies, selected by a selection unit and fed to the knitting systems of the machine by a positive feeding device.
As outlined above, known knitting machines of this type are specifically designed for the production of striped fabrics and are rather complicated and bulky.
The invention provides a simpler production unit for striped fabrics and is characterised in that the knitting machine is a conventional knitting machine having no selection units immediately associated with its knitting systems, that a yarn selection unit controlled in synchronism with the working cycle of the knitting machine and in dependence on the desired striping pattern is provided separate from each knitting system, that a yarn knotting, clamping and cutting device is associated with each yarn selection unit, that an intermediate yarn storage device is provided downstream of the knotting, clamping and cutting device, and that a positive yarn feeding device is provided downstream of the intermediate yarn storage device.
Preferably, the intermediate yarn storage device consists of a stationary drum and a winding-on element provided with a winding drive. In this way, the additional twist on the unbroken yarn is kept at a minimum. It would, however, also be possible to use a rotating intermediate storage drum.
Preferably, the positive yarn feeding device is a tape feeder. Such a tape feeder can feed all the unbroken yarns to the knitting system of the machine and is of simple and rugged structure.
A further task underlying the invention is to provide a simple and yet efficient control for the feeding of the unbroken yarns to the knitting systems in such a way that the desired striping pattern is exactly produced.
To this end the invention provides a control system consisting of a patterning unit controlling the selection unit and a secondary control unit controlling the knotting, clamping and cutting device and the drive for the winding-on element, said secondary control unit being associated with first and second sensors sensing the yarn lengths consumed by the knitting machine and fed into the intermediate yarn storage device and supplying the secondary control unit with such yarn length information.
Since the secondary control unit always contains information on the length of yarn present in the intermediate storage device and on its way from said storage device to the knitting system, and since the secondary control unit controls the knotting, clamping and cutting device, the lengths of the differently coloured yarn portions can very exactly be determined and controlled so that a regular striping pattern develops.
Preferably, the secondary control unit controls the knotting, clamping and cutting unit and the drive for the winding-on element in such a way that the drive is inactive when the knotting takes place. This greatly facilitates the joining together of the yarn ends in the knotting device.
In the preferred embodiment of the knitting machine according to the invention a third sensor is associated with the secondary control unit sensing the working position of the knitting machine and supplying the secondary control unit with such machine position information.
This additional information enables the control system to provide for a continuous correction of the yarn joining operation and of the length measurement of the yarn to exactly position the knots in the fabric and to compensate for imperfections in the drive system of the knitting machine.
An embodiment of the invention is disclosed schematically in the attached drawings and will be described hereinafter: